The detection of gases and vapors at low concentrations is often difficult due to limitations in the sensitivity of detector devices and measurement instruments. The process of detecting gases and vapors at low concentrations can be greatly enhanced if the gas or vapor can be concentrated prior to detection. Concentration of toxic materials, contaminants or other substances involving the utilization of a sorbent material which selectivity sorbs and desorbs the toxic material, contaminants, etc. is, in and of itself, known in the art. Typically, upon desorption the toxic substance, contaminant, etc. is then conducted to a sensor device or measurement instrument which registers the presence of the contaminant.
One example of such chemical measurement technology is gas chromatography. In this technique, vapors present in a flowing stream of carrier gas are forced to flow through a tube, the wall of which is coated with a sorbent material. The vapors absorb and desorb from the coating as they migrate through the tube. Usually, vapors of different chemical composition spend different amounts of time absorbed into the coating with the result that the vapors are desorbed at different times, and thus exit the tube after different elapsed times. This can afford extremely good selectivity in separating chemical compounds having similar physical and chemical properties. Furthermore, the vapors elute from the tube in a brief pulse. Thus, a detector positioned at the outlet of the tube is subjected to a sudden change in concentration of the vapor in question which is easily discriminated from slow detector signal variations (known as "drift") caused by temperature changes, impurities in the carrier gas, etc. However, the gas chromatographic approach to enhancing detection selectivity is rather disadvantageous in that a significant amount of time must be allowed for all of the vapors to elute from the tube and thus be detected, one following the other. Gas chromatography affords selectivity, but unfortunately by detecting and analyzing in a time-serial fashion, which is very time-consuming.
In a paper entitled "Quartz Crystal Gas Monitor With Gas Concentrating Stage", Kindlund, A., Sundgren, H., and Lundstrom, Ingemar; Sensors and Actuators, 6 (1984) pp. 1-17, there is described a recent alternative development. A coated channel is placed ahead of a sensor. Gases are collected on the channel's coating and thin thermally desorbed to improve sensor selectivity, sensitivity and drift performance. However, the apparatus described by the authors uses a single preconcentration channel, and only a single sensor. Thus, as with gas chromatography, chemical selectivity is achieved in a time-serial fashion, which is disadvantageous. Additionally, the apparatus described in the above-mentioned article was physically quite large; a Peltier element was employed in association with the preconcentration channel to provide heating and cooling for the purposes of effecting desorption of collected gases. This approach is accordingly further disadvantageous in that it requires the consumption of a substantial amount of power.
Chemical sensors, especially chemical microsensors, potentially afford many attractive features to the art such as low cost, high sensitivity, ruggedness and (in the case of microsensors) small size. These features are important in many applications. In certain embodiments, these chemical sensors are being utilized in combination with one another to make up an array of sensors. Tis array of sensors can be coupled to a pattern recognition processor to enhance the operational selectivity of the sensor system. These pattern recognition processor systems employ a pattern recognition algorithm to analyze data fed to the processor from the array of sensors when those sensors come in contact with chemical species which it is desired to detect. The analysis of information obtained from such chemical sensors with a pattern recognition processor using a pattern recognition algorithm as an analysis technique is described for example, in Pattern Recognition Principals, Tou, J. T., Gonzalez, R. C., Addision-Wesley, Redding, Mass. (1974); The Interpretation of Analytical Chemical Data by Use of Cluster Analysis, Massart, D. L., Kaufman, L. John Wiley, New York, New York (1983); Carey W. P., Beebe, K. R., Kowalski B. R., Illman, D. L., Hirschfeld, T., Analytical Chemistry, 1986, Vol. 58, p. 149 et seq. Nevertheless, as can be seen from the foregoing discussion, to date the art has not taken appropriate advantage of this powerful analytical tool in the monitoring of gas, and detection and identification of gas species which may be present in said gas.